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Colbert, E.H., 1942. Notes on the lesser one-horned rhinoceros, Rhinoceros sondaicus, 2. The position of Rhinoceros sondaicus in the phylogeny of the genus Rhinoceros. American Museum Novitates 1207: 1-5, figs. 1-3

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Location: World
Subject: Taxonomy - Taxa
Species: Asian Rhino Species


Original text on this topic:
It might be well at this place to indicate the distinctions in the skull, jaw and dentition between the two recent species, Rhinoceros unicornis and Rhinoceros sondaicus, since many of these distinctions are of considerable importance in the following discussion of the phylogenetic position of each species in relation to the other. Moreover, by calling attention to these osteological differences between the two modern species it may be that some aid will be afforded to workers in the future who may have occasion to examine skulls and teeth of Rhinoceros.
Flower (1876) published a paper in which he described many of the distinguishing traits between the two living species of Rhinoceros, while Osborn in his monograph on 'The Extinct Rhinoceroses' (1898) discussed briefly certain differences to be seen between the living species of Rhinoceros, and in addition presented a very useful comparative figure in which lateral views of the skulls and mandibles of all the living rhinoceroses were shown to scale. Upon the basis of these previously published works and original observations of the American Museum specimens of Rhinoceros, the accompanying comparison has been drawn up (see p. 2).
Skull, Mandible and Dentition
,Rhinoceros unicornis,Rhinoceros sondaicus
1,Large and robust,Smaller and lighter than R. unicornis
2,Nasals expanded into large, rounded horn boss,Less expansion in the nasals; horn boss pointed rather than rounded, and very small in female
3,Ascending ramus very high,Ascending ramus not extremely heightened
4,Occipital surface high and narrow. Skull deep.,Occipital surface comparatively low and broad. Skull comparatively shallow.
5,Deep 'saddle' in profile of skull, between nasals and occipital vertex,Rather shallow saddle in cranial profile.
6,Zygomatic arch rounded at posterior termination,Zygomatic arch angular at posterior termination
7,Posterior margin of palate concave, or with small median projection,Posterior margin of palate with median projection
8,Mesopterygoid fossa, basisphenoid and basi-occipital bones narrow,Mesopterygoid fossa, basisphenoid and basi-occipital bones comparatively broad
9,Pterygoids compressed and grooved,Pterygoids flattened and laterally expanded
10,Vomer thick and unted to sides of pterygoid processes,Vomer thin, lamelliform, pointed and free
11,Premaxillaries broad,Premaxillaries comparatively narrow
12,Teeth strongly sub-hypsodont,Teeth less hypsodont than R. unicornis
13,Ectoloph of cheek-teeth rather flat,Ectoloph of cheek teeth sinuous
14,Parastyle buttress suppressed,Parastyle buttress prominent
15,Well developed crochet and crista, united in worn tooth to enclose a medifossette,Crochet present but crista generally absent
This comparison of the characters in the two modern species of Rhinoceros is interesting in that it shows what might be called a 'harmonic' specialization of Rhinoceros unicornis over Rhinoceros sondaicus. The one species is advanced beyond the other not by virtue of a few isolated characters but in all features throughout the structure of the, skull, jaws and dentition; every character listed above for the distinction of the two forms shows an advance in the Indian rhinoceros over its expression in the Javan form. It is not possible on the basis of the material available to compare skeletons of the two species, but it would seem likely that much the same picture would hold in a lesser degree in the post-cranial region. Certainly the external characters in the two species conform to this conception of a general, all-round specialization of Rhinoceros unicornis over Rhinoceros sondaicus.

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